January 2016

These microfilaria were found on direct wet mount of whole blood from a dog from Texas with no history of heartworm preventive use. Each microfilaria measures approximately 300μ long by 6-7μ wide. A heartworm antigen test run on the same sample was negative. What parasite is this most likely to be, and how can you explain the negative antigen test?

What is your diagnosis? Scroll down to see the answer...

Answer:

​​These are microfilaria of Dirofilaria immitis, the canine heartworm. The identity of these microfilaria can be confirmed by their length, width, and lack of progressive movement in a wet mount of whole blood. As many as 5-10% of dogs from the southern United States infected with heartworm may test negative on antigen tests. In the present case, heat pre-treatment of plasma was done and the antigen test repeated. The sample was strong positive for heartworm antigen following heat treatment.

When antigen tests for D. immtis were first developed, pre-treatment to destroy immune complexes was commonly done prior to testing samples for antigen but this step is no longer routinely recommended. Antigen can usually be detected following heat treatment of serum or plasma from dogs with microfilaria of D. immitis that are false negative on an initial antigen test. Other explanations for microfilaria positive, antigen negative samples include microfilaria other than D. immits (e.g. Acanthocheilonemareconditum), a history of treatment for adult heartworms without clearance of microfilaria, and transfusion from a microfilaremic donor dog. Pups born to a microfilaremic dam may also occasionally have microfilaria of D. immitis.